Delay device for use in transmission of oscillations



Patented Nov. 25, 1941 ST? .S FAT DELAY'DEVECE FOR USE IN'TRANSMKSSION F OSCILLATION S of GreatBritain.

Application February 2, 1939,.Serial.No'..254 ,l 71 In Great-Britain February 8, 1-938 5 Claims.

This inventionv relates to delay devices and is of general application: in obtaining delay in the transmission of electric oscillations representing signal or controlling voltages.

High frequency signalling transmitting and receiving systems sometimes include delay networks which are required for example when the signals are passed through separate channels, the outputs of which are subsequently combined and the time of transmission through one channel is different from that in the other. In order that the transmission times of the separate channels shall be equal, a delay network is placed in the channel having the shorter transmission time.

Again, in the interference.- suppression system described in the specification of British Patent #468,994, for Improvements in. and relating. to the reduction of interference in electrical signal transmission systems, accepted July- 8, 1937, an auxiliary channel supplied with interfering signals provides a controling voltage which serves to switch off the main signal channel and in order that the controlling voltage shall be applied to the main channel at the desired in?- stant, the delay in the auxiliary channel due to the selective and voltage generating circuits, is compensated in the main channel by the provision of a delay network capable of passing a band of signal frequencies.

In low frequency volume compression or expansion systems, it is necessary to generate a controlling voltage proportional to the compression effect introduced and in such systems delay in the auxiliary circuits provided, again has to be compensated.

In the specification of U. S. Patent application Serial No. 223,911, filed Sept. 8, 1938, for Signaling system, by William S. Percival, now U. S. Pat. #2219936, patented Oct. 29, 1949, there is described a wave filter system in which provision is made for selecting from a range of signal frequencies a frequency band of given width and subsequently including said selected band within said range, the output from a filter employed to elfect the selection of the smaller band being subtracted from said range. The whole of the frequencies in the range are in certain cases delayed before the subtraction is effected by a period equal to the time delay of the filter, The delay networks employed may consist of filter sections including inductance and capacity in known combinations.

The object of the present invention is to provide apparatus for producing delay in the transmission of high; frequency signals and according to the invention an. electric time delay device comprises a: quartz crystal. for generating supersonic waves in aliquid through which saidwaves are transmitted and then' detected by another quartz crystal, the delay in transmissionv being determined by the. distance. through the. liquid between the crystals. Water or kerosene have been found to be suitable liquids for the. purpose of the invention. The crystalsshould; have the same or substantially. the: same resonant frequency and provision may be made. to ensure an unimpeded path through the liquid. between the crystals in order to-reduce or avoid. reflection of the supersonic waves.

The liquid container may be tubular: in shape or narrow in one dimension and wide in another or wide in both dimensions. The effective length of the liquid container will, of course, depend upon the amount of delay required. If any of the dimensions other than those of the length are not small compared with the length it is desirableto take steps to reduce reflection of the waves passing through the-liquid. Such refiection can be reduced oreliminated by covering the walls of'the liquid container with a suitable absorbent material such as wool or' cork or alternatively the insideof the liquid container may have a number of narrow" slots or holes formed thereinso that thewaves passing into the slots or holes are rapidly absorbed; Again a number of sheets of" absorbent material may be hung in the liquid so as to leave clear a straight line between the quartz crystals.

In order to obtain a large delay in a comparatively small volume of liquid the container may be divided into a number of compartments, space being left alternatively at each end of the container so as to provide reflecting surfaces for the supersonic waves which are thus reflected into the adjacent compartments. Such a construction is shown diagrammatically in the accompanying drawing in which I represents a liquid container, 2 a tortuous space in the container in which kerosene or water is placed, quartz crystals 3 and 5 being arranged at 0pposite ends of the space 2. The approximate path of travel of the supersonic waves generated by the quartz crystal 3 to the crystal 4 is shown by the dotted line 5.

Assuming a velocity of the supersonic waves of 10 ems/sec. a distance through the liquid of 10 cms. will give a delay of micro seconds. The delay may, of course, be varied by adjusting the position of one or other of the quartz crystals 3 and 4 along the length of the path 2.

The delay obtained by the use of the device described can be advantageously utilised for most purposes by employing a carrier wave of, for eX- ample, 10 megacycles per second which carrier wave can be modulated by the signal it is desired to delay. The use of a modulated carrier wave is desirable owing to the relatively rapid change of attenuation with frequency and owing to the resonant character of the quartz crystals. If a wider frequency band is desired than can be obtained by means of the natural damping of the quartz crystals by the liquid then the natural frequency of the two crystals can be staggered to obtain a relatively flat response over a wide pass-band. The device described enables delays to be obtained of the order of several hundred micro seconds with a pass-band of one megacycle or more.

A particular application of the invention lies in the system described in the specification of co-pending U. S. Patent application Serial No. 228,911, now Patent No. 2,235,018, March 18,

1941, the device taking the place of the delay networks consisting of filter sections including inductance and capacity as indicated in that specification. The device can, however, replace any delay network when a long time delay covering a wide pass-band is required. In such cases it will generally be desirable to include either a modulating and demodulating device respectively before and after the delay in order to change the frequency band to within a region of the order of, for example, 10 magacycles. an oscillator may be used to heterodyne the signal to be delayed to the required frequency.

' I claim:

1. An electric time delay device comprising a first piezo-electric crystal for generating supercated at the other end of said delay path, a plurality of partitions and reflecting surfaces within said structure to form an indirect time delay path, and means within the Walls of said structure for absorbing undesired reflection of said waves passing through said liquid.

Alternatively,

2. The method of obtaining a large time delay in the transmission of electric oscillations covering a wide band of frequencies which consists in transmitting said oscillations through a liquid indirect path between a transmitting and a receiving quartz crystal source, the fluid volume of said path being small as compared with the length of the time delay path which is determined by the distance through the liquid between the transmitting source and the receiving source.

3. An electric time delay device comprising a casing member, a liquid within said casing, a pair of quartz crystals located within said casing member, one of said crystals arranged for generating supersonic waves in said liquid through which said waves are transmitted and subsequently detected by the other quartz crystal, a labyrinth having slanting side walls creating a liquid relatively long indirect path of varying volume and located within said casing, the time delay in transmission being determined by the distance through said liquid path, labyrinth and between the crystals.

4. An electric time delay device comprising a casing member, a liquid within said casing, a labyrinth having slanting side walls creating a liquid relatively long path of varying volume and located within said casing, a pair of quartz crystals located within said casing, said crystals having substantially the same resonant frequency as the waves which are to be transmitted, one of said crystals arranged for generating the frequency to be transmitted through said liquid path contained Within said labyrinth, the other crystal being arranged to receive said waves, the time delay being determined by the distance through said liquid path within the labyrinth and between the crystals.

5. An electric time delay device comprising a first piezo-electric crystal for generating Waves in a liquid, a second piezo-electric crystal for detecting said Waves, a casing member, a plurality of partitions within said casing which form a tortuous space to provide a relatively long indirect liquid path, the opposed surfaces of said partitions being shaped to act as reflectors to ensure unimpeded reflection of the transmitted waves from said first and second piezo-electric crystals.

- WILLIAM SPENCER PERCIVAL. 

